The interaction between grapefruit juice and calcium-channel blockers such as felodipine, is the most dramatic example of drug-diet interactions reported to date. It is widely assumed the interaction is caused by bioflavonoids present in the juice inhibiting the main enzyme involved in metabolism of felodipine (CYP3A4). These studies, performed in collaboration with the FDA, will attempt to identify the specific components in grapefruit juice responsible for this diet-drug interaction.